Roller conveyors are used to move packages, trays, or containers where accumulation of individual containers is required. Roller conveyors may be advantageously used when items are accumulated from a number of locations and dispatched to multiple destinations. For example in moving sorted mail contained in trays from sorting locations to a loading dock where mail destined for a particular vehicle or container is accumulated. Roller conveyors consist of a roller tube which is mounted for rotation about the cylindrical axis of the roller. The rollers are spaced on the conveyors so that the minimum sized package is supported by at least three rollers at all times. Drive rollers are positioned between idler rollers and used to move and stop packages on the conveyor. The spacing and number of drive rollers relative to idler rollers depends on the size of the packages anticipated and the torque or drive force which is required. With groups of drive rollers may be used if greater drive forced is required.
Although drive rollers may be driven by belts from external motors, the typical modern approach is to use rollers with internal electric motors which are mounted to a fixed shaft and drive the roller through a gear system. The use of internal-motor-driven rolls simplifies the assembly of the roll conveyor and eliminates a large number of pulleys and external motors which occupy space and require guards to protect workers.
Roller conveyors can be used to precisely control the motion of many packages with each motor under the control of a central computer or programmable controller. By controlling the individual drive motors a package can be moved forward or can be brought to a stop by using the drive rollers as a brake. The rollers can operate individually or as an integrated system to accumulate packages with zero pressure, and release packages in a slug release or singulation mode. A roller conveyor can be used to combine streams of packages onto a single conveyor going to a single or multiple destinations. Packages on a first conveyor line which intersects a second conveyor line can be held and injected one by one as space becomes available on the second conveyor. Thus the conveyors will typically be used with multiple sensors which sense the passage of discrete packages, and a controller which is designed and programmed for a particular application.
Standard rollers of both the drive, and idler types are 1.9 inches in diameter. The drive rollers are typically supplied as a sealed unit and utilize small high speed brush motors. The roller sealed units are typically disposed of when the brushes wear out. The typical life of a drive roller and motor unit is less than 2,000 hours. A typical drive roller costs $90.00 and has a service life of less than one year. Although units using brushless D.C. motors are known, the life of the units is still limited by the life of the gears which drive the roller tube.
Cost is a very important consideration in the design of drive rollers for roller conveyors. A typical installation may contain hundreds to thousands of feet of conveyor and, with one drive roller spaced every two to three feet, the number of drive rollers which must be purchased yearly to maintain such a system can be a considerable expense.
What is needed is a drive roller with a longer life and a lower cost.